Improving the ablation resistance of C/C composites under ultra-high temperature environments is a prerequisite for their applications as thermal protection components in high-speed aircraft. When conventional silicide-modified C/C composites serve in an aerobic environment at >1700 ℃, the active oxidation of silicide is intensified, causing the formation and escape of gaseous SiO from unoxidized silicide. Concurrently, the decomposition pressure and saturation vapor pressure of SiO₂ increase with temperature, accelerating their decomposition and volatilization from the oxide scale. These processes lead to a rapid consumption of silicide, preventing the formation of a dense and continuous oxygen barrier, and resulting in a gradual loss of thermal protection capability.

Among ultrahigh-temperature ceramics (UHTCs), HfC and ZrC are preferred modified components for their high melting points (>3000 ℃) and good chemical stability, as well as high melting points of their oxidation products (i.e., HfO₂: 2810 ℃ and ZrO₂: 2677 ℃). Nevertheless, their protective oxide scales exhibit a loose structure after long-term oxidation/ablation due to the lack of a densification pathway capable of healing oxidation/ablation-induced defects (pores and cracks). To mitigate this issue, the liquid-phase sintering effect of low-melting oxide phases (i.e., TiO₂: ~1840 ℃, Ta₂O₅: ~1800 ℃, and Nb₂O₅: ~1512 ℃) promotes their solid solution process with high-melting HfO₂/ZrO₂ and produces dense Hf/Zr-X-O compound (X=Ti, Ta and Nb), healing ablative defects during ablation. As a result, the ablation resistance of the coatings is significantly improved. Consequently, some multi-phase carbides are widely applied for C/C composites, including HfC-ZrC-TiC, HfC-TaC, ZrC-TaC, HfC-ZrC-TaC, HfC-NbC, and ZrC-NbC. However, they are typically prepared by mechanical blending through ball milling before spraying, showing an inhomogeneous elemental distribution in a microscale. It induces locally aggregated low-melting phases, having an insufficient mechanical denudation resistance, thus impeding further improvement of ablation resistance. Conventional silicide- and multi-phase carbide ceramic-modified C/C composites face some challenges, including insufficient high-temperature stability, limited long-term ablation resistance, and narrow protective temperature ranges.

In comparison to these ceramics, medium-/high-entropy carbide (M/HEC) ceramics unique "four effects" and tunability of composition and microstructure endow excellent comprehensive properties. This review represented the preparation methods of M/HEC ceramics, which are composed of solid-phase reaction (i.e., Carbothermal reduction method, Direct synthetic method, and Molten salt synthesis) and liquid-phase reaction (i.e., Polymer-derived ceramics and Sol-gel) methods, and discussed their effects on the structure and properties of M/HEC ceramics. The preparation methods of M/HEC substrate-modified C/C (i.e., Precursor infiltration and pyrolysis, Reaction melt infiltration, and Molten salt infiltration methods) and M/HEC coating-modified C/C (i.e., Supersonic atmospheric plasma spraying method) were described, and the preparation cycles of different processes and their effects on the structure and properties of C/C composites were compared. Finally, the influence of different M/HEC on the ablation resistance was summarized, and some promising prospects for the future development of M/HEC substrate/coating-modified C/C composites were proposed.